Carbon fibers are a new breed of
high-strength materials. Carbon fiber has been described as a fiber containing
at least 90% carbon obtained by the controlled pyrolysis of appropriate fibers.
The existence of carbon fiber came into being in 1879 when Edison
took out a patent for the manufacture of carbon filaments suitable for use in
electric lamps. However, it was in the early 1960s when successful commercial
production was started, as the requirements of the aerospace industry -
especially for military aircraft - for better and lightweight materials became
of paramount importance. In recent decades, carbon fibers have found wide
application in commercial and civilian aircraft, recreational, industrial, and
transportation markets. Carbon fibers are used in composites with a lightweight
matrix. Carbon fiber composites are ideally suited to applications where
strength, stiffness, lower weight, and outstanding fatigue characteristics are
critical requirements. They also can be used in the occasion where high
temperature, chemical inertness and high damping are important. The suppliers
of Advanced Composites Materials Association released 1997 industry
statistics on worldwide shipments of carbon fibers for composites [1,2](Table
1). However, from 1997 to 1999 there was a global slowing of carbon fiber
demand [3]. According to Mitsubishi Rayon Co. Ltd. (Tokyo, Japan),
a carbon fiber producer, worldwide consumption for sporting goods is nearly 11
million lb of carbon fiber

Table
1: Worldwide shipment of carbon fibers for composites

Year

Pounds

1992

13,000,812

1993

14,598,544

1994

17,425,452

1995

19,714,671

1996

20,672,741

1997

25,900,000

Currently, the United States of America uses
nearly 60% of the world production of carbon fibers and the Japanese account
for almost 50% of the world capacity for production. The largest producer of
this fiber is Toray Industries of Japan. The world production
capacity of pitch-based carbon fiber is almost totally based in Japan
[4].

Table
2: Us composite Shipment in 1998

Market

Percent of total volume

Transportation

31.6

Construction

20.8

Corrosion-
resistant

11.8

Marine

10.1

Electrical/Electronics

10.0

Consumers

6.3

Appliances/Business
equipment

5.5

Aircraft

0.6

Others

3.3

2. CLASSIFICATION AND TYPES:

Based on modulus, strength, and final heat treatment
temperature, carbon fibers can be classified into the following categories:

2.1 Based on
carbon fiber properties, carbon fibers can be grouped into:

2.3 Based on
final heat treatment temperature, carbon fibers are classified into:

·Type-I, high-heat-treatment carbon fibers (HTT),
where final heat treatment temperature should be above 2000°C and
can be associated with high-modulus type fiber.

Type-II,
intermediate-heat-treatment carbon fibers (IHT), where final heat
treatment temperature should be around or above 1500°C
and can be associated with high-strength type fiber.

Type-III,
low-heat-treatment carbon fibers, where final heat treatment temperatures
not greater than 1000°C. These are low modulus and
low strength materials.

3. MANUFACTURE

In Textile Terms and Definitions, carbon fiber
has been described as a fiber containing at least 90% carbon obtained by the
controlled pyrolysis of appropriate fibers. The term "graphite fiber"
is used to describe fibers that have carbon in excess of 99%. Large varieties
of fibers called precursors are used to produce carbon fibers of different
morphologies and different specific characteristics. The most prevalent
precursors are polyacrylonitrile (PAN), cellulosic fibers (viscose rayon,
cotton), petroleum or coal tar pitch and certain phenolic fibers.

Carbon fibers are manufactured by the controlled
pyrolysis of organic precursors in fibrous form. It is a heat treatment of the
precursor that removes the oxygen, nitrogen and hydrogen to form carbon fibers.
It is well established in carbon fiber literature that the mechanical
properties of the carbon fibers are improved by increasing the crystallinity
and orientation, and by reducing defects in the fiber. The best way to achieve
this is to start with a highly oriented precursor and then maintain the initial
high orientation during the process of stabilization and carbonization through
tension.

4. Carbon
fibers from POLYACRYLONITRILE (PAN):

There are three successive stages in the conversion of
PAN precursor into high-performance carbon fibers. Oxidative stabilization: The
polyacrylonitrile precursor is first stretched and simultaneously oxidized in a
temperature range of 200-300°C. This treatment converts thermoplastic PAN to a
non-plastic cyclic or ladder compound. $Carbonization: After oxidation, the
fibers are carbonized at about 1000°C without tension in an inert atmosphere (normally
nitrogen) for a few hours. During this process the non-carbon elements are
removed as volatiles to give carbon fibers with a yield of about 50% of the
mass of the original PAN. Graphitization: Depending on the type of fiber
required, the fibers are treated at temperatures between 1500-3000°C,
which improves the ordering, and orientation of the crystallites in the
direction of the fiber axis.

4.1.1 THE CONVERSION OF RAYON FIBERS INTO CARBON
FIBERS IS THREE PHASE PROCESS

Stabilization: Stabilization is an oxidative
process that occurs through steps. In the first step, between 25-150°C,
there is physical desorption of water. The next step is a dehydration of the
cellulosic unit between 150-240°C. Finally, thermal cleavage of the cyclosidic linkage
and scission of ether bonds and some C-C bonds via free radical reaction
(240-400°
C) and, thereafter, aromatization takes place.

Carbonization: Between 400 and 700°C,
the carbonaceous residue is converted into a graphite-like layer.

Graphitization: Graphitization is carried out
under strain at 700-2700°C to obtain high modulus fiber through longitudinal
orientation of the planes.

Fig.
2: Reactions involved in the conversion of cellulose into carbon fibers

4.1.2. The carbon fiber fabrication from pitch
generally consists of the following four steps:

Pitch preparation: It is an adjustment in the
molecular weight, viscosity, and crystal orientation for spinning and further
heating.

Spinning and drawing: In this stage, pitch is
converted into filaments, with some alignment in the crystallites to achieve
the directional characteristics.

Stabilization:
In this step, some kind of thermosetting to maintain the filament shape during
pyrolysis. The stabilization temperature is between 250 and 400 °C.

Carbonization: The carbonization temperature is
between 1000-1500°C.

Fig. 3:
Manufacturing process schematic for pitch-based carbon fibers

5. CARBON FIBERS IN MELTBLOWN NONWOVENS

Carbon fibers made from the spinning of molten pitches
are of interest because of the high carbon yield from the precursors and the
relatively low cost of the starting materials. Stabilization in air and
carbonization in nitrogen can follow the formation of melt-blown pitch webs.
Processes have been developed with isotropic pitches and with anisotropic
mesophase pitches. The mesophase pitch based and melt blown discontinuous
carbon fibers have a peculiar structure. These fibers are characterized in that
a large number of small domains, each domain having an average equivalent
diameter from 0.03mm to 1mm and a nearly unidirectional orientation of folded
carbon layers, assemble to form a mosaic structure on the cross-section of the
carbon fibers. The folded carbon layers of each domain are oriented at an angle
to the direction of the folded carbon layers of the neighboring domains on the
boundary [5].

6. Carbon
fibers from isotropic pitch:

The isotropic pitch or pitch-like material, i.e.,
molten polyvinyl chloride, is melt spun at high strain rates to align the
molecules parallel to the fiber axis. The thermoplastic fiber is then rapidly
cooled and carefully oxidized at a low temperature (<100°C).
The oxidation process is rather slow, to ensure stabilization of the fiber by
cross-linking and rendering it infusible. However, upon carbonization,
relaxation of the molecules takes place, producing fibers with no significant
preferred orientation. This process is not industrially attractive due to the
lengthy oxidation step, and only low-quality carbon fibers with no
graphitization are produced. These are used as fillers with various plastics as
thermal insulation materials.

7. Carbon
fibers from anisotropic mesophase pitch:

High molecular weight aromatic pitches, mainly
anisotropic in nature, are referred to as mesophase pitches. The pitch
precursor is thermally treated above 350°C to convert it to
mesophase pitch, which contains both isotropic and anisotropic phases. Due to
the shear stress occurring during spinning, the mesophase molecules orient
parallel to the fiber axis. After spinning, the isotropic part of the pitch is
made infusible by thermosetting in air at a temperature below it's softening
point. The fiber is then carbonized at temperatures up to 1000°C.
The main advantage of this process is that no tension is required during the
stabilization or the graphitization, unlike the case of rayon or PAN
precursors.

8. STRUCTURE

The characterization of carbon fiber microstructure has
been mainly been performed by x-ray scattering and electron microscopy
techniques. In contrast to graphite, the structure of carbon fiber lacks any
three dimensional order. In PAN-based fibers, the linear chain structure is
transformed to a planar structure during oxidative stabilization and subsequent
carbonization. Basal planes oriented along the fiber axis are formed during the
carbonization stage. Wide-angle x-ray data suggests an increase in stack height
and orientation of basal planes with an increase in heat treatment temperature.
A difference in structure between the sheath and the core was noticed in a
fully stabilized fiber. The skin has a high axial preferred orientation and
thick crystallite stacking. However, the core shows a lower preferred
orientation and a lower crystallite height.

9. PROPERTIES

In general, it is seen that the higher the tensile
strength of the precursor the higher is the tenacity of the carbon fiber.
Tensile strength and modulus are significantly improved by carbonization under
strain when moderate stabilization is used. X-ray and electron diffraction
studies have shown that in high modulus type fibers, the crystallites are
arranged around the longitudinal axis of the fiber with layer planes highly
oriented parallel to the axis. Overall, the strength of a carbon fiber depends
on the type of precursor, the processing conditions, heat treatment temperature
and the presence of flaws and defects. With PAN based carbon fibers, the
strength increases up to a maximum of 1300oC and then gradually
decreases. The modulus has been shown to increase with increasing temperature.
PAN based fibers typically buckle on compression and form kink bands at the
innermost surface of the fiber. However, similar high modulus type pitch-based
fibers deform by a shear mechanism with kink bands formed at 45° to
the fiber axis. Carbon fibers are very brittle. The layers in the fibers are
formed by strong covalent bonds. The sheet-like aggregations allow easy crack
propagation. On bending, the fiber fails at very low strain.

10. APPLICATIONS

The two main applications of carbon fibers are in
specialized technology, which includes aerospace and nuclear engineering, and
in general engineering and transportation, which includes engineering
components such as bearings, gears, cams, fan blades and automobile bodies.
Recently, some new applications of carbon fibers have been found. Such as
rehabilitation of a bridge [6] in building and construction industry. Others
include: decoration in automotive, marine, general aviation interiors, general
entertainment and musical instruments and after-market transportation products
[7]. Conductivity in electronics technology provides additional new
application. Table 2 illustrates some of the characteristics and applications
of carbon fibers[8].

The production of highly effective fibrous carbon
adsorbents with low diameter, excluding or minimizing external and
intra-diffusion resistance to mass transfer, and therefore, exhibiting high
sorption rates is a challenging task. These carbon adsorbents can be converted
into a wide variety of textile forms and nonwoven materials [9]. Cheaper and newer versions of carbon
fibers are being produced from new raw materials. Newer applications are also
being developed for protective clothing (used in various chemical industries
for work in extremely hostile environments), electromagnetic shielding and
various other novel applications. The use of carbon fibers in Nonwovens is in a
new possible application for high temperature fire-retardant insulation (eg:
furnace material.)